Referring to FIGS. 1 and 2, a conventional vane device using wind or water energy to drive a power generator (not shown) that generates electricity is shown. The conventional vane device includes a flow guide casing 11 and a rotary unit 12 rotatably mounted on the flow guide casing 11. The flow guide casing 11 includes two end covers 111, an inner casing wall 112 that is disposed below the rotary unit 12 and that is curved to have a curvature corresponding to a moving orbit of the rotary unit 12, and an outer casing wall 113 that is disposed below the inner casing wall 112 and that extends forward and upward and thereafter bends rearward. The rotary unit 12 includes a rotary shaft 121 rotatably mounted on tops of the end covers 111, and a plurality of vanes 122 extending radially and outwardly from the rotary shaft 121.
Wind currents higher than the rotary shaft 121 (referred to as positive direction wind currents 13 hereinafter) drive the vanes 122 to rotate the rotary shaft 121 in a rotation direction 14 for generation of electricity. The inner casing wall 112 guides wind currents lower than the rotary shaft 121 (referred to as bottom wind currents 15 hereinafter) to drive rotation of the vanes 122 in the rotation direction 14. The end covers 111 and the inner casing wall 112 bound a flow guide passage 114. The flow guide passage 114 diverts the bottom wind currents 15 to follow the rotation direction 14 for driving the vanes 122. The bottom wind currents 15 are therefore efficiently utilized. However, when the bottom wind currents 15 flow upward after driving the vanes 122 and meet the positive direction wind currents 13, the amount of the positive direction wind currents 13 is increased, and a large amount of the positive direction wind currents 13 are pushed upward as depicted in FIG. 2 so that they are unable to effectively act on the vanes 122. Therefore, the efficiency of the conventional vane device for electrical power generation is reduced.